1. Field of the Invention
The present invention relates to a lattice array-structured piezoelectric actuator having multiple piezoelectric actuator elements arrayed in a lattice pattern, in particular, to such a lattice array-structured piezoelectric actuator integrated with a flexible printed circuit board having a large number of electric signal lines, and to a method for producing the same.
2. Description of the Related Art
The recent tendency in the art of actuators is toward a high-density array structure, or that is, a matrix array structure of tabular piezoelectric actuator elements disposed on a substrate. With it, the electric signal input terminals of the circuit board terminal unit for the recent actuators are also to have a matrix array structure.
In an ordinary actuator, in general, a common electrode and a signal electrode are formed on the two major faces of each of such matrix-arrayed, multiple tabular piezoelectric actuator elements on a substrate. The common electrode is electrically connected with the substrate, and electric signals from a control circuit are applied to the tabular piezoelectric actuator elements via the substrate and via the common electrode and the signal electrode, thereby motivating the tabular piezoelectric actuator elements to move in the direction vertical to the two major faces thereof.
For electrically bonding the tabular piezoelectric actuator elements to the circuit board, heretofore employed is a method of once rearranging the conductor matrix array of tabular piezoelectric actuator elements into a one-dimensional array thereof and thereafter bonding the thus-rearranged conductor terminals to the electric signal output terminals of an external driving unit to thereby finish the intended actuator, in view of the production costs and the productivity of the method.
FIG. 19 and FIG. 20 are perspective views showing the constitution of matrix-arrayed piezoelectric actuators according to the related art, mentioned as above. FIG. 19 shows one constitutional example of such actuators in which every actuator element is electrically bonded to a circuit board in a mode of wire bonding (this is hereinafter referred to as a related art 1); and FIG. 20 shows another constitutional example thereof in which every actuator element is electrically bonded to a circuit board in a mode of screen printing (this is hereinafter referred to as a related art 2).
First described is the matrix-arrayed piezoelectric actuator of the related art 1 shown by FIG. 19. As illustrated, this has tabular piezoelectric actuator elements 101, a substrate 102, printed circuit boards 103, and wires 104. On both the two major faces of each of the matrix-arrayed tabular piezoelectric actuator elements 101 on the substrate 102, formed is a thin electrode film, and one of the two major faces thereof is bonded to the substrate 102. Every wire 104 is bonded to any of the printed circuit boards 103, and to the thin electrode film formed one major face of each tabular piezoelectric actuator element 101. The printed circuit boards 103 each are made to electrically communicate with a control circuit (not shown) that selectively imparts an electric signal to each tabular piezoelectric actuator element 101. Accordingly, the electric signal is imparted to each tabular piezoelectric actuator element 101 via the printed circuit boards 103.
Next described is the matrix-arrayed piezoelectric actuator of the related art 2 shown by FIG. 20. As illustrated, this has tabular piezoelectric actuator elements 101, a substrate 102, a flexible printed circuit (FPC) board 106, and a conductor wiring pattern 105. On both the two major faces of each of the matrix-arrayed tabular piezoelectric actuator elements 101 on the substrate 102, formed is a thin electrode film, and one of the two major faces thereof is bonded to the substrate 102. In the area except the sections in which each tabular piezoelectric actuator element 101 is bonded to the substrate 102, all the actuator elements 101 are completely electrically insulated from the substrate 102. The conductor wiring pattern 105 is screen-printed on the substrate 102, extending from the thin electrode film formed on the other major face of every tabular piezoelectric actuator element 101 to the flexible printed circuit board 106. The thin electrode film formed on each tabular piezoelectric actuator element 101 is electrically bonded to the flexible printed circuit board 106 by heating the conductor wiring pattern 105 and the flexible printed circuit board 106 under pressure via a solder put therebetween. The flexible printed circuit board 106 is made to electrically communicate with a control circuit (not shown) that selectively imparts an electric signal to each tabular piezoelectric actuator element 101. Accordingly, the electric signal is imparted to each tabular piezoelectric actuator element 101 via the flexible printed circuit board 106.
One example of the related art 2 is proposed in JP-A-4-77257, which teaches a method of electrically bonding matrix-arrayed, multiple tabular piezoelectric actuator elements formed on a substrate to a flexible printed circuit board by once rearranging them into a one-dimensional matrix array conductor pattern through screen-printing on the substrate followed by electrically bonding the thus-rearranged one-dimensional pattern to the flexible printed circuit board.
JP-A-11-300956 (corresponding to U.S. Pat. No. 6,190,006) discloses another technique of separately bonding the individual piezoelectric elements to a flexible printed circuit board in fabricating an actuator. This is a related art 3. In this, provided are segment terminal electrodes each individually neighboring piezoelectric vibrators corresponding thereto, and the thickness of each segment terminal electrode is made larger than that of each piezoelectric vibrator to thereby ensure a space between the flexible printed circuit board and the piezoelectric vibrators so as to prevent any mutual mechanical contact of the actuator elements therein.
The actuator of the type mentioned above has not one but multiple matrix-arrayed piezoelectric actuator elements, in which various physical phenomena such as vibration or heat generation of the individual actuator elements make the actuator elements interfere with each other through the substrate, or they have some influences on the substrate itself and thereby have significant influences on the action of the individual actuator elements. Accordingly, it must be considered that the matrix-arrayed actuator has a circuit board as a part thereof.
However, the method of electrically bonding a thin electrode film formed on each tabular piezoelectric actuator element to a printed circuit board by bonding every wire extending from each actuator element to the printed circuit board, like the related art 1, is unfavorable to high-density matrix structures, since the height and the length of the wire loops are limited and therefore the number of the terminals capable of being bonded to the printed circuit board is limited. Another problem with the method is that the terminals must be bonded to the printed circuit board one by one, and the total process to finish the bonding operation takes too much time.
In the related art 1, all the bonded area must be sealed up with resin or the like for preventing the wires from touching the neighboring ones and from absorbing moisture to thereby evade electric short-circuits or insulation failure therein. However, the resin sealing is problematic in that it restricts the displacement level of the tabular piezoelectric actuator elements and therefore the actuator could not have the designed displacement characteristics.
On the other hand, the method of electrically bonding matrix-arrayed, multiple tabular piezoelectric actuator elements formed on a substrate to a flexible printed circuit board by once rearranging them into a one-dimensional matrix array conductor pattern through screen-printing on the substrate followed by electrically bonding the thus-rearranged one-dimensional pattern to the flexible printed circuit board, like the related art 2, is also unfavorable to high-density matrix structures, since the minimum wiring pattern width in screen printing therein is limited. For example, when tabular piezoelectric actuator elements each having an electrode-forming face size of 0.5 mm×0.5 mm are arrayed in a matrix size of 10×10 actuator elements, the minimum wiring pitch in screen printing is generally limited to 0.3 mm or so. In that case, therefore, the matrix pattern is limited to a pitch of 3.65 mm or so.
In the related art 2, the matrix conductor pattern is once rearranged to a one-dimensional pattern. In this, therefore, the region in which the thus-rearranged one-dimensional pattern is electrically bonded to a flexible printed circuit board shall increase with the increase in the matrix density (that is, the increase in the number of the patterned wiring lines), and, as a result, the probability of bonding failure occurrence exponentially increases, and the device reliability is difficult to ensure according to this method. Another problem with the method is that the productivity is low and the production costs are therefore high according to it.
In both the related arts 1 and 2, the direction of the printed circuit board in which the board is settled in the actuator is naturally limited to only the side opposite to the matrix array of tabular piezoelectric actuator elements in order to ensure the space region that corresponds to the electric bonding site of the substrate, and, as a result, the region above the tabular piezoelectric actuator elements shall be a dead space to lower the bonding efficiency of the flexible printed circuit board to the matrix-arrayed tabular piezoelectric actuator elements, and, in addition, the electric bonding itself of the printed circuit board to the matrix-arrayed actuator elements is extremely difficult. This is still another problem with these arts.
In the related arts 1 and 2, in addition, the vibration and the heat generation to be caused by the individual actuator elements' motion may often break the constitutive components of the actuator or may often cause some unstable motion of some actuator elements. This detracts from the practicability of the actuator, and this is still another problem with these arts.
The related art 3 is free from the problems with the related arts 1 and 2. However, this requires the segment terminal electrodes each individually corresponding to the piezoelectric vibrators therein, and its problem is that the production costs in this art are high.